What are Reference intervals?

This short animation explains reference intervals and how they are set.
You may have noticed that some of your test results are shown in your report as a comparison against a set of numbers called reference interval or reference ranges. This is the interval or range of test results that has been deemed ‘normal’ for the general population - most healthy people can be expected to have results that fall within this interval.
How do we determine what is normal for a diagnostic test? The first step in defining a reference interval is to define the ‘healthy’ population. Reference intervals are established by taking the results of large numbers of healthy people and calculating what is typical for them. Factors such as age, sex and ethnicity can affect results, so often a series of reference intervals is established.
The reference interval for a particular laboratory test is expressed as the average value for the ‘normal’ population group together with the variation around that value (plus or minus 2 standard deviations from the average). In this way, intervals represent the values found in 95% of individuals in the reference group. A consequence of this approach is that even in a ‘normal’ population a test result is outside the reference range in 5% of cases (1 in 20).
It’s important to realise that having a result that is marked ‘high’ or ‘low’ and is outside the reference interval is not necessarily something you need to be alarmed about. Your result should be considered within the context of your personal circumstances and with the benefit of your own and your doctor's knowledge of your past medical and personal history, together with the results of any other investigations performed.